It is known, in particular from French patent specification FR-2 214 873A, to provide a plate-type heat exchanger for effecting heat transfer between a first fluid, for example an oil to be cooled, and a second fluid, for example a coolant liquid, and comprising:
a casing having an inlet pipe and an outlet pipe for the second fluid;
a stack of plates arranged in pairs in alternating relationship within the casing, around a central tube defining a stacking direction, in such a way that the plates of any one said pair define between them a flow channel for the first fluid, with the pairs of plates defining between them, and within the casing, flow channels for the second fluid which are in communication with each other; and
communication means adapted for flow of the first fluid between the successive pairs of plates from an inlet for the first fluid to an outlet for the first fluid.
In known heat exchangers of this type, the plates have the general shape of a disc which usually has an internal circular lip and a peripheral circular lip, with the lips extending in a plane at right angles to the stacking direction. Thus, when the similar lips of two adjacent plates, which together constitute one pair of plates, are joined together, for example by brazing, these two plates define an annular channel between them for the flow of the first fluid, which may for example be an oil.
In order to allow the first fluid to flow from one pair of plates to another, each plate has two flow apertures (or ports) diametrically opposed to each other, namely an inlet aperture and an outlet aperture, with each of these apertures (or ports) being bounded by a lip which is arranged to be joined sealingly to a similar lip of an adjacent plate.
Such heat exchangers are used most particularly for the cooling of lubricating oil received from the cylinder block of an engine. In the central tube, around which the disc-shaped plates are mounted, a threaded stud is engaged. This stud serves firstly for securing the heat exchanger on to the engine, and secondly for securing an oil filter on the heat exchanger itself. The hollow central tube also provides a path for the return of the cooled oil to the engine, either directly within the tube or through the threaded stud, which is then made hollow for this purpose.
In addition, in a known heat exchanger of the above mentioned type, a bypass is commonly provided. This bypass has a flap valve which is normally open when the oil is cold and viscous, but closed when the oil is hot and fluid. In the opening position of the flap valve, the oil flows directly through the heat exchanger from the oil inlet of the latter to the bypass, by passing through the inlet apertures of the plates in the stack, so as to arrive directly at the filter, from which it returns to the motor through the central tube or stud. In that event, the oil is not cooled. When the flap valve is closed, on the other hand, the oil is distributed into each flow passage in the stack of plates via the inlet apertures of the plates, and it leaves each of these flow passages through the outlet apertures in the plates from which it passes to a passage which communicates with the filter. It is returned to the engine through the central tube or hollow stud. Under these conditions the oil is cooled by heat transfer with the cooling liquid.
Known types of heat exchanger of the kind described above have various disadvantages, due to the fact that the plates of any one pair are assembled together through flat lips which extend at right angles to the stacking direction. This method of assembly first of all reduces the useful heat exchange surface of each plate. In addition, sealing between the first and second fluid is obtained firstly by means of the central tube, and secondly through contact between two flat surfaces which are defined in relief on the two adjacent plates of two different pairs of plates. In addition to this, in the event of failure of the heat exchanger, a leak can occur which leads to mixing of the first fluid with the second fluid.